CephFS Administrative commands


These commands operate on the CephFS filesystems in your Ceph cluster. Note that by default only one filesystem is permitted: to enable creation of multiple filesystems use ceph fs flag set enable_multiple true.

fs new <filesystem name> <metadata pool name> <data pool name>
fs ls
fs rm <filesystem name> [--yes-i-really-mean-it]
fs reset <filesystem name>
fs get <filesystem name>
fs set <filesystem name> <var> <val>
fs add_data_pool <filesystem name> <pool name/id>
fs rm_data_pool <filesystem name> <pool name/id>


fs set <fs name> max_file_size <size in bytes>

CephFS has a configurable maximum file size, and it’s 1TB by default. You may wish to set this limit higher if you expect to store large files in CephFS. It is a 64-bit field.

Setting max_file_size to 0 does not disable the limit. It would simply limit clients to only creating empty files.

Maximum file sizes and performance

CephFS enforces the maximum file size limit at the point of appending to files or setting their size. It does not affect how anything is stored.

When users create a file of an enormous size (without necessarily writing any data to it), some operations (such as deletes) cause the MDS to have to do a large number of operations to check if any of the RADOS objects within the range that could exist (according to the file size) really existed.

The max_file_size setting prevents users from creating files that appear to be eg. exabytes in size, causing load on the MDS as it tries to enumerate the objects during operations like stats or deletes.

Taking the cluster down

Taking a CephFS cluster down is done by setting the down flag:

mds set <fs_name> down true

To bring the cluster back online:

mds set <fs_name> down false

This will also restore the previous value of max_mds. MDS daemons are brought down in a way such that journals are flushed to the metadata pool and all client I/O is stopped.

Taking the cluster down rapidly for deletion or disaster recovery

To allow rapidly deleting a file system (for testing) or to quickly bring MDS daemons down, the operator may also set a flag to prevent standbys from activating on the file system. This is done using the joinable flag:

fs set <fs_name> joinable false

Then the operator can fail all of the ranks which causes the MDS daemons to respawn as standbys. The file system will be left in a degraded state.

# For all ranks, 0-N:
mds fail <fs_name>:<n>

Once all ranks are inactive, the file system may also be deleted or left in this state for other purposes (perhaps disaster recovery).


Most commands manipulating MDSs take a <role> argument which can take one of three forms:


Comamnds to manipulate MDS daemons:

mds fail <gid/name/role>

Mark an MDS daemon as failed. This is equivalent to what the cluster would do if an MDS daemon had failed to send a message to the mon for mds_beacon_grace second. If the daemon was active and a suitable standby is available, using mds fail will force a failover to the standby.

If the MDS daemon was in reality still running, then using mds fail will cause the daemon to restart. If it was active and a standby was available, then the “failed” daemon will return as a standby.

tell mds.<daemon name> command ...

Send a command to the MDS daemon(s). Use mds.* to send a command to all daemons. Use ceph tell mds.* help to learn available commands.

mds metadata <gid/name/role>
mds repaired <role>
mds stat

Global settings

fs dump
fs flag set <flag name> <flag val> [<confirmation string>]

“flag name” must be one of [‘enable_multiple’]

Some flags require you to confirm your intentions with “–yes-i-really-mean-it” or a similar string they will prompt you with. Consider these actions carefully before proceeding; they are placed on especially dangerous activities.


These commands are not required in normal operation, and exist for use in exceptional circumstances. Incorrect use of these commands may cause serious problems, such as an inaccessible filesystem.

mds compat rm_compat
mds compat rm_incompat
mds compat show
mds set_state
mds rmfailed


These legacy commands are obsolete and no longer usable post-Luminous.

mds add_data_pool # replaced by "fs add_data_pool"
mds cluster_down  # replaced by "fs set cluster_down"
mds cluster_up  # replaced by "fs set cluster_up"
mds dump  # replaced by "fs get"
mds getmap # replaced by "fs dump"
mds newfs # replaced by "fs new"
mds remove_data_pool # replaced by "fs rm_data_pool"
mds set # replaced by "fs set"
mds set_max_mds # replaced by "fs set max_mds"
mds stop  # obsolete